The present invention relates generally to methods for treating a subterranean formation in order to stabilize swelling clays and migrating fines.
The recovery of fluids such as oil and gas from subterranean formations has been troublesome in formations that contain water sensitive minerals, e.g., water-swellable clays such as clays in the smectite group and fines capable of migrating when disturbed such as silica, iron minerals and alkaline earth metal carbonates. Clay minerals ubiquitous in sedimentary rocks are hydrous aluminum phyllosilicates characterized by a stack of two-dimensional sheets composed of the SiO4 tetrahedron and/or the AlO4 octahedron. They can cause major problems during hydrocarbon exploration as clay minerals have a tendency to swell/migrate in the presence of water. The water can come from injection fluids, drilling muds, stimulation fluids, and gels. The ion-exchangeable cations present in-between the aluminosilicate sheets attract water in the interlayer molecular spaces and cause swelling. For example, the smectite group of clays such as montmorillonite absorbs water and expands in volume. During oil exploration, introduction of water causes these clays to swell, ultimately degrading the porosity and reducing the formation permeability. The clay mineral kaolinite, on the other hand, generates fines in the presence of water and tends to migrate. These fines migrate through the formation, blocking the pore spaces and reducing the permeability of the formation.
This loss in permeability and plugging or impairing the flow of formation fluids toward the well bore results in either a loss of the formation fluids or a decrease in the rate of recovery from the well. Sometimes the migrating fines are produced with the formation fluids and present abrasion and other problems with the above-ground equipment.
In an effort to overcome these problems, various methods have been developed for treating subterranean formations to stabilize swelling clays and migratable fines. For example, it has been a common practice to add salts to aqueous drilling fluids. The salts adsorb to clay surfaces in a cation exchange process and can effectively reduce the swelling and/or migration of the clays. Various polymers and consolidating resins have also been used. However, in many areas, environmental regulations restrict the use of high salt concentrations and various resin and polymer compositions. Thus, there is a continuing need for improved and more environmentally benign methods and compositions for treating subterranean formations to prevent or reduce the swelling of clays and the migration of fines during drilling, treating and fluid recovery operations.